Fix Accept parsing

There is a bug in the comma-splitting logic for Accept (not fixed in #196). It doesn't ignore commas inside quoted strings. A correct splitter would need to identify `,`, `"`, and `\"`. See [Spring's tokenizer](https://www.google.com/url?q=https://github.com/spring-projects/spring-framework/blob/7bbd4c681fd009898a62cac6e11eae77039a4119/spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java%23L274&sa=D&source=hangouts&ust=1548433024564000&usg=AFQjCNFkqlocOoRDdAL9J8mObQTnU_fTBw) for example.

This bug is mentioned in `accept_test.go`, but downplayed as rare. However, it is likely to occur in the future when Chrome advertises support for `v="b3,b4"`.

Start with accept_test.go and reproduce the Accept header case containing a quoted comma, such as v="b3,b4". Compare the comma-splitting behavior with Spring's tokenizer reference, then verify that commas inside quoted strings and escaped quotes are handled correctly. Done means the regression case passes without breaking ordinary Accept parsing.

Written by the indexing model from the issue text.